Over the past year hackers have increasingly focused on Office applications as a security hole and the attacks based on these flaws are often the same: Criminals send an email that appears legitimate, and trick unsuspecting users into opening a maliciously encoded document. Once the document is opened, attackers are able to install unauthorised software on the victim's system.

Microsoft may not have time to patch the Excel flaw in time for its next set of security updates, expected 13 February, but, no doubt, it has other Office patches in the works. Over the past few months four similarly critical flaws have been reported in Microsoft Word. None of these has yet been patched.